Cancer accounts for ˜23% of all deaths each year and is the second leading cause of death in the United States. Because of its high mortality and preceding morbidity rates, cancer has a significant social and economic impact upon our society. Given that five-year survival rates drastically plummet with tumor stage at the time of detection, there is a pressing public health need to develop and rigorously validate sensitive and robust non-invasive imaging biomarkers capable of identifying tumors at the earliest possible time and to predict patients likely to respond to therapeutic interventions.
Thus, there is a critical need to develop and rigorously validate quantitative biomarkers capable of interrogating the underlying pathophysiology of tumors, as well as to predict the clinical outcome of therapeutic interventions. Assay of conventional cancer biomarkers typically requires invasive procurement of limited quantities of tumor tissue with attendant risks and sampling errors due to heterogeneity. Furthermore, serial tumor biopsies, as are required to assess treatment response longitudinally, are clinically impractical in many instances. Non-invasive imaging circumvents these limitations and offers potential advantages over traditional biopsy-based procedures. Imaging techniques routinely used in clinical oncology include magnetic resonance imaging (MRI), x-ray computed tomography (CT), ultrasound imaging (US), and PET. Of these, the sensitivity and quantitative nature of PET, coupled with the ability to readily produce biologically active compounds bearing positron emitting isotopes, renders PET one the most attractive techniques for detecting tumors and profiling their genetic and molecular features. Despite this, the number of PET tracers currently available for profiling tumors, and accordingly the diversity of biological questions addressable with PET, is limited. By far the most widely used PET tracer in oncology is [18F]FDG, a probe that measures glucose utilization and established tool for cancer diagnosis and staging. However, [18F]FDG has important limitations, including modest uptake in some tumors (e.g. prostate) and elevated background uptake in certain normal tissues (e.g. brain). Furthermore, a plethora of metabolic processes affect [18F]FDG uptake, highlighting a currently unmet need to explore and validate additional molecular targets for cancer imaging.
Formerly referred to as peripheral benzodiazepine receptor (PBR), TSPO is an 18 kDa high-affinity cholesterol- and drug-binding protein that participates in regulation of numerous cellular processes, including cholesterol metabolism, steroid biosynthesis, proliferation and apoptosis. Elevated TSPO expression is well documented in neuroscience and oncology. To date, many preclinical and human studies have shown that tumors arising in the breast, prostate, oral cavity, colon, liver, and brain can express high levels of TSPO, suggesting a role for this molecule in carcinogenesis. Given the role of TSPO in regulation of proliferation and Bcl-2-mediated apoptosis, it is not surprising that TSPO expression tends to correlate with tumor proliferation and aggressive, invasive tumor behavior. Clinically, TSPO levels predict metastatic potential, disease progression and diminished survival in patients with breast, oral, colorectal, and brain tumors.
Embodiments of the present invention include candidates from aryloxyanilides, pyrazolopyrimidines, indoleacetamides, and indolylglyoxylylamides that exhibit significant improvements over classic TSPO ligands such as the isoquinoline carboxamide, PK 11195. While a few of these ligands have been developed as PET imaging probes for neuroscience applications, they have not been explored in cancer imaging.
Malignant gliomas are the most common primary brain tumor and are characterized by invasive growth and recalcitrance to therapy. Currently, diagnosis and grading of gliomas are based upon the pathology of resected specimens with limitations inherent to sampling errors and heterogeneity. Given these limitations, clinical decisions are routinely guided by imaging. The most common imaging metrics employed to detect and diagnose brain tumors are computed tomography (CT) and magnetic resonance imaging (MRI). These modalities provide little, if any, molecular information attributable to the pathological status of the disease. Furthermore, numerous studies document the inherent difficulty associated with determination of brain tumor extent using CT and/or MRI, particularly with infiltrative disease. Positron emission tomography (PET) imaging using [18F]FDG is an important technique for brain tumor detection, however, high glucose uptake in normal brain results in modest tumor-to-background ratios, which can confound delineation of disease margins and subsequent grading. Therefore, there is a considerable need to develop and validate improved molecular imaging techniques suitable for detection and/or molecular profiling of brain tumors.
TSPO is typically localized to the outer mitochondrial membrane. TSPO participates in regulation of numerous cellular processes, including cholesterol metabolism, steroid biosynthesis, cellular proliferation, and apoptosis. In normal tissues, TSPO expression tends to be highest in steroid-producing and mitochondrial-enriched tissues such as skeletal muscle, renal tissue, and myocardium, while tissues such as liver and brain exhibit comparatively modest expression. Elevated TSPO expression is found in numerous disease states, including neuroinflammation and psychiatric disorders such as Alzheimer's and Huntington's diseases, as well as cancers of the breast, prostate, oral cavity, colon, liver, and brain. Elevated TSPO expression has also been linked with disease progression and diminished survival in patients with oral, colorectal, breast, and brain cancer. Additionally, elevated TSPO levels appear to be associated with aggressive, metastatic behavior in breast, colorectal, and prostate cancer. Collectively, these data illuminate TSPO expression as a potentially important prognostic biomarker in oncology and suggest the potential utility of tumor-selective TSPO PET ligands for cancer imaging.
Clinically, two of the most common imaging metrics employed to detect and diagnose brain tumors are computed tomography (CT) and magnetic resonance imaging (MRI). These modalities provide little, if any, molecular information attributable to the pathological status of the disease. Furthermore, numerous studies document the inherent difficulty associated with visualization of the true extent of brain tumor pathology using CT and/or MRI, particularly with highly infiltrative disease. Positron emission tomography (PET) using [18F]FDG is among the most powerful imaging approaches currently available for tumor detection in nearly all organ sites, including the brain. However, high glucose uptake in normal brain results in modest tumor-to-background ratios, which can confound delineation of disease margins and subsequent grading. An alternative and potentially superior approach to [18F]FDG PET is L-[methyl-11C] methionine ([11C]methionine). Though promising, the half-life of 11C limits the broad implementation of this technique. Therefore, there is a considerable need to develop and validate improved molecular imaging techniques suitable for detection and/or molecular profiling of brain tumors.
Given its elevated expression and correlation with aggressive tumor phenotypes, cellular proliferation, and grade in glioma, imaging TSPO expression in brain tumors has been suggested and explored previously. Almost exclusively, these studies employed the well-known TSPO ligand, (R)—N—[3H/11C]methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)-isoquinoline-3-carboxamide), [3H/11C](R)-PK 11195, for either autoradiographic methods or PET imaging. These studies highlighted important limitations of PK 11195 as a molecular imaging probe. For example, despite the fact that TSPO expression can be up to 12-fold higher in brain tumors compared to normal brain, [11C](R)-PK 11195 uptake was shown to be relatively modest in tumors compared to normal brain tissue (≦2:1 T/N,). The high degree of non-displaceable PK 11195 binding documented in both normal brain and tumors limits the ability of [11C](R)-PK 11195 to directly reflect TSPO expression.